Christopher Watts, the Colorado man who is serving a life sentence for killing his pregnant wife and two daughters, confessed in a prison interview that his daughters were still alive when he put his dead wife’s body in his truck and drove his family to the oil field where all three bodies were found, according to an investigative report and audio recording of the interview released on Thursday.

During a nearly five-hour interview with investigators last month, Mr. Watts, 33, revealed that he killed his family over a longer period of time in August than was previously reported and painted a disturbing picture of his daughters’ final moments.

He said that he strangled his wife, Shanann Watts, after a fight at their home in Frederick, Colo., and that his 4-year-old daughter, Bella, witnessed him dragging the body down the stairs.

“What is wrong with Mommy?” Bella cried, according to his account.

He then loaded Bella and her 3-year-old sister, Celeste, into the back seat of his truck, with their mother’s body nearby on the floorboard, and drove them about 45 minutes to the oil site, he said.

When he got there, he said, he used a blanket to smother Celeste, who was nicknamed “Cece,” and dropped her in an oil tank, before doing the same to Bella.

“Is the same thing gonna happen to me as Cece?” Bella asked before she died, according to his account.

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Christopher Watts at his arraignment in Greeley, Colo., in August. He was sentenced in November to life in prison without parole.CreditPool photo

The revelation that the Watts girls died after a prolonged episode that ended at the oil site, rather than at home, was the latest turn in an evolving account from Mr. Watts, who went on television soon after his wife and children vanished and said he did not know where they were.

“Shanann, Bella, Celeste, if you’re out there, just come back,” Mr. Watts said on Aug. 14 in an interview with Denver7. “If somebody has her, just bring her back. I need to see everybody.”

He was arrested the next day.

Mr. Watts, who the authorities found had been having an affair with a co-worker, initially sought to pin the blame on his wife, who he said was upset after he told her he wanted to separate, according to an arrest warrant affidavit.

He claimed that he had seen his wife strangling Celeste via the baby monitor and that Bella was lying on her bed and appeared blue. He said he went into a rage and strangled Ms. Watts before disposing of all three bodies at the oil site.

In his interview with investigators last month, he maintained that he and his wife had an emotional conversation on the night of her death. When he told her he wanted to end their marriage, he said, she said he would never see the children again — so he strangled her.

He said he came up with the idea to blame Ms. Watts for the girls’ deaths during his interrogation by the police. When the possibility that Ms. Watts might have killed the girls was mentioned, he said, he “just went with it.”

But he acknowledged the truth to his lawyers a few weeks later, he said.

In November, he pleaded guilty to nine criminal counts — including murder, the unlawful termination of a pregnancy and tampering with a dead body. He was sentenced that month to life in prison without the possibility of parole.

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Mementos outside the home where the Watts family lived in Frederick, Colo.CreditDavid Zalubowski/Associated Press

Lawyers from the Colorado public defender’s office, which represented Mr. Watts, did not respond to requests for comment about the prison interview, including questions about whether Mr. Watts’s account was accurate and why he decided to share the information now.

Mr. Watts told investigators he wanted his wife’s family to have closure.

Grant & Hoffman Law Firm, which is representing Ms. Watts’s family, declined to comment, citing an exclusive agreement with the television show “Dr. Phil.” Ms. Watts’s parents, Sandra and Frank Rzucek, and her brother, Frankie Rzucek, are scheduled to appear on the show Monday.

In an interview that aired this week, Steven Lambert, a lawyer for the firm, told Dr. Phil that he believed the latest account from Mr. Watts.

“The only reason he might lie about it is if he was protecting somebody,” he said.

Mr. Lambert said it had been particularly difficult for Ms. Watts’s mother to imagine the final moments for Bella, who prosecutors said “fought back for her life.”

“She wonders if in Bella’s last moments — seeing as she saw her mother being wrapped up in a sheet, she saw her sister being killed — if some of her last thoughts might have been: ‘Where’s Grandma? Where’s Grandpa?’” Mr. Lambert said.

Thomas D. Grant, another lawyer for the firm, told Dr. Phil that, amid a case that has drawn national attention, the Rzucek family hoped people would remember that Ms. Watts, Bella and Celeste were “real people” who died at the hands of a family member.

“They want people to know: This was our daughter. These were our grandchildren,” he said. “We loved them.”